As is known in the art, plasma is an ionized gas, in which electrons heated by an electric field are responsible for ionizing gas atoms. At a low gas pressure, the hot electrons inside a plasma have relatively few collisions with the gas atoms. Therefore, the gas remains cool, as one observes in a fluorescent light (p˜1 Torr). At or near atmospheric pressure (p˜760 Torr), however, the free electrons in the plasma frequently collide with gas atoms and heat the gas to very high temperatures (e.g., 5,000-10,000 K). Examples of atmospheric plasmas include lightning and welding arcs. High temperature plasmas tend to be destructive and are unsuitable for many industrial processes, including photo-voltaic manufacturing.
Recently, plasma generators have been developed that produce plasma that is relatively low-temperature at or near atmospheric pressure. These low-temperature, atmospheric-pressure plasmas are known as “cold” plasmas, and are characterized by their lower gas temperatures, often less than 500° K. and generally in the range of 300-1000° K. These cold plasma discharges are not constricted arcs but are typically quite small (<1 mm) and do not cover relatively broad areas of up to 1 m2 as can be required for industrial processes. These low-temperature atmospheric-pressure plasmas, however, are advantageous for numerous industrial processing applications, and in particular for processing inexpensive commodity materials that are sensitive to heat, such as plastics.
An example of a microplasma generator for generating cold plasma at atmospheric pressure is a split ring resonator (SRR). In this device, the microplasma is generated in a discharge gap, e.g., 25 μm, formed in a ring-shaped microstrip transmission line. The cold atmospheric plasma is generated by coupling microwave energy (0.4-2.4 GHz) to plasma electrons using a resonating circuit. The circuit generates high electric fields (E˜10 MV/m) that heat the plasma electrons without strong coupling to the rotational and vibrational modes of the gas molecule, i.e., without generating significant heat. The gas temperature within the plasma can be measured using the rotational spectra of nitrogen molecules and is typically in the range of 100-400° C. Exemplary embodiments of SRR plasma generators are described in U.S. Pat. No. 6,917,165 to Hopwood et al., the entire contents of which are incorporated herein by reference for all purposes.
Known microplasma generators employ a microwave resonating circuit to generate a low-temperature atmospheric-pressure plasma. Of the known cold plasma technologies, the microwave resonator approach offers the most intense electron density while maintaining the lowest gas temperature and the longest electrode life.
One drawback to the existing cold plasma generators is that their geometries are not optimized for some industrial processing, particularly processes for altering the surface of a substrate. The SRR device, for example, is limited to a single “point” geometry, that severely limits its effectiveness for processing a wide-area substrate. Quarter-wave microstrip resonators have been demonstrated to generate microplasmas and can be assembled into linear arrays. These arrays do not scale well to sizes of industrial interest, however, as at larger linear array sizes plasma might not be generated by the resonators near either edge of the array.
What is needed, therefore, is a device for generating a microplasma that can be better controlled and tuned for specific applications and that can provide plasma over a larger area.